The model below shows a carbide ion.

8 light grey and 6 dark grey balls sit at the center with 2 concentric black rings around them. The inner ring has 2 medium grey balls on it, and the outer ring has 8 medium grey balls on it. At bottom, a dark grey ball = +1, a light grey ball = 0, a medium grey ball = negative 1.

What could be done to change this carbide ion to a neutral carbon atom?

remove 2 electrons
add 2 electrons
remove 4 electrons
add 4 electrons

it is the minimum amount of material needed to sustain a fission reaction.

critical mass is the minimum amount of fissionable material that must be assembled for a sustained fission reaction to take place. although two to three neutrons are produced for every fission, not all of these neutrons are available for continuing the fission reaction. if the conditions are such that the neutrons are lost at a faster rate than they are formed by fission, the chain reaction will not be self-sustaining.   at the point where the chain reaction can become self-sustaining, this is referred to as critical mass.